Two motors that require 19.2 Amps each at 24 volts, Then a total of 5 amps of other electronics at 12 volts. This is fed by a DC-DC converter that outputs 10 amps 12 volts. So I was thinking 45 amps would be safe in case of any current spikes.

Two motors that require 19.2 Amps each at 24 volts, Then a total of 5 amps of other electronics at 12 volts. This is fed by a DC-DC converter that outputs 10 amps 12 volts. So I was thinking 45 amps would be safe in case of any current spikes.

Get an automotive 24V relay that can handle at least 50A.You can get them with higher capacity if you like, but a good quality 50A relay should do - Hella makes them among others

Then use any switch you like to switch the relay.

Automotive fuses at the positive terminal of your battery is cheap and a really good idea - one fo each circuit (at least a large current fuse for the motors and a low current fuse for the rest).

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

Could I use a 24V 40 amp relay such as this one because the only electronics that are running at 24 volts is the motors and I just read that the motors actually run at 18 amps each. All of the other electronics run at 12 volts. On the second link it says the max continuous current is 50 amp. Do you think I'll be ok?

Could I use a 24V 40 amp relay such as this one because the only electronics that are running at 24 volts is the motors and I just read that the motors actually run at 18 amps each. All of the other electronics run at 12 volts. On the second link it says the max continuous current is 50 amp. Do you think I'll be ok?

In that case, just use a 12V relay. The voltage of an automotive relay (AR) is the coil voltage and the current rating is for the contacts and they don't care if you put 12V or 24V on them.

I recently stumbled over 12V AR's at around $2.00 to $2.50.Considering that many AR's is made in China or places with just as "strict a quality control", I would keep a wide margin.An easy way to do that is using two AR's, one for each motor (still with one switch) and I have drawn the connections for two (and an ON indicator).What's drawn with dotted lines can be discarded if you only want to use one AR and no indicator (LED).

The numbers on the relay terminals are usually shown on an AR, so it's easy to find out which goes to the coil terminals and which are for the contacts (neither the coil nor the contacts are polarized, so can be reversed if it helps the wiring).

If I were to use two relays, I would need two 20 amp relays, right?Also, would I put this in between the Sabertooth and each of the motors if I was using two relays?

What switch what I need then? 10 amps?

Would I need diodes able to hand 20 amps then? And would I just put the diode in between the diode leads or am I reading that incorrectly? I have never used a diode for this purpose so I am not sure how I would rate it.

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Swell, but you still need fuses - to protect you, your home and your electronics!

I'm sure there's something along the line of "Fuses... We don't need no stinkin' fuses" in some collection of Famous Last Words

Also, would I put this in between the Sabertooth and each of the motors if I was using two relays?

They are best placed before the Sabretooth, i.e. right after the 24V battery, but the relay contacts just replace a mechanical switch, so anywhere you would place a switch (wheter one or two relays are used).

And would I just put the diode in between the diode leads or am I reading that incorrectly? I have never used a diode for this purpose so I am not sure how I would rate it.

No need to rate anything, just follow the schematic I posted

I intended to show the wiring as it should be, with the voltage to the relay sort of passing the diode. When you remove the voltage to the relay, it will generate a reversed polarity high voltage pulse (a dampened oscillation really) and this is when the diode go to work, by shorting out this pulse. That is why there should be as short a distance between the relay col terminals and the diode and why the diode should be the first thing the kick back sees.

It would work without the diode, but not for very long, as the switch contacts would either weld together or get pitted so much that it cannot deliver even the relatively small coil current to the relay.With the diode in place, the (negative) pulse will stay under 1V. Without it, the pulse will be somewhere between -500V and -1000V.

They are best placed before the Sabretooth, i.e. right after the 24V battery, but the relay contacts just replace a mechanical switch, so anywhere you would place a switch (wheter one or two relays are used).

If I were to use two relays and connect them before the Sabertooth, how would I do that? What would be the point of the using two if your not even connecting them to each motor?

What? That doesn't make any sense. I asked a question about how to use the relays and you tell me to use just one switch and then you tell me Soeren explained how to use the relays.

I still dont understand how that would work if I put the relays before the Sabertooth. Why would you use two switches for the Sabertooth? What would be the point of the using two if your not even connecting them to each motor?

What? That doesn't make any sense. I asked a question about how to use the relays and you tell me to use just one switch and then you tell me Soeren explained how to use the relays.

Ok, confusion is taking over. In Your previous post You mentioned that 50A switches are hard to find, hence I reminded that You can use automotive relay with small switch to controll it (as Soeren adviced).

I still dont understand how that would work if I put the relays before the Sabertooth. Why would you use two switches for the Sabertooth? What would be the point of the using two if your not even connecting them to each motor?

If You put relay as main switch, then it is placed just after the battery and You need only one. If You want to switch (ON/OFF) every motor separately, two relays are added - one relay in series with one motor (between motor and Sabertooth).

Also, You did not specify which Sabertooth You are using, 1ch or 2ch. If You use 1ch Sabertooth - for 2 motors, 2 drivers are required,hence You need two switches (relays) no matter where they are to be placed (before or after motor drivers).

Hope this makes it clear

« Last Edit: November 03, 2011, 03:49:47 PM by newInRobotics »

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Ok, confusion is taking over. In Your previous post You mentioned that 50A switches are hard to find, hence I reminded that You can use automotive relay with with small switch to controll it (as Soeren adviced).

Ok I understand what you were trying to say

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f You put relay as main switch, then it is placed just after the battery and You need only one. If You want to switch (ON/OFF) every motor separately, two relays are added - one relay in series with one motor (between motor and Sabertooth).

Yeah I dont want to independently control each motor with relays because theat's the Sabertooth 2x25's job

They are best placed before the Sabretooth, i.e. right after the 24V battery, but the relay contacts just replace a mechanical switch, so anywhere you would place a switch (wheter one or two relays are used).

If I were to use two relays and connect them before the Sabertooth, how would I do that? What would be the point of the using two if your not even connecting them to each motor?

Economics... You linked to a (40A IIRC) relay costing close to $9, while you might get 20..25A relays for around $2.00 to $2.50

Just parallel the switches if you want them cutting a single wire and you have a combined rating of twice the current. The motor controller won't draw very much current when you don't send a signal to it, so the contacts will stabilize before the high power is drawn and you shouldn't cut the switch while the motors go full speed anyway, so when switching, the current will be fairly low.

If you feel more for a single relay, go ahead, but if you want the relay(s) to stay healthy for a long time (i.e. not developing too much contact resistance too fast), I'd advice at least a 50A combined rating.

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

Thank you for the nice explanation. I'm no expert, but this all sounds too complicated for a simple switch. What do you think about a light switch? I saw one at Home depot that was rated 30 amps at 120 VDC. That has to be able to withstand40 amps at 24 volts. I'm not sure how to calculate the amperage the switch would be rated at for 24 volts.

What do you think about a light switch? I saw one at Home depot that was rated 30 amps at 120 VDC. That has to be able to withstand40 amps at 24 volts.

Don't count on it...The voltage rating of a switch is about isolation.The current rating starts out as a power rating, but since the (max) contact resistance is known, the current can be found by the formula: I=sqr(P/R)

Assuming that the 30A switch has got a contact resistance of say 0.01 Ohmn (10 mOhm), any voltage from 0.3V up to whatever voltage the contact separation allows without flash over will dissipate the same power (9W in this case).

The only difference you'll see is when switching and switching an inductive really lowers what a contact can handle, but at least you can switch with no power going to the motors and then you're in the clear as long as the contacts are rated for a fair bit over the anticipated current.

I'm not sure how to calculate the amperage the switch would be rated at for 24 volts.

It is simply bad economy to buy a switch that is just rated for what current it will see. A good quality proper rated switch may berated eg. 50% higher and be 50% more expensive, but if it lasts four times as long, it's still the cheapest solution.You could do away with the switch by using the fuse as switch - just get automotive fuses of the type that connects to spade terminals.

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

Don't count on it...The voltage rating of a switch is about isolation.The current rating starts out as a power rating, but since the (max) contact resistance is known, the current can be found by the formula: I=sqr(P/R)

Assuming that the 30A switch has got a contact resistance of say 0.01 Ohmn (10 mOhm), any voltage from 0.3V up to whatever voltage the contact separation allows without flash over will dissipate the same power (9W in this case).

The only difference you'll see is when switching and switching an inductive really lowers what a contact can handle, but at least you can switch with no power going to the motors and then you're in the clear as long as the contacts are rated for a fair bit over the anticipated current.

Ok thank you for this piece of information. I definitely need this switch to last so I think I'll go with something else.

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You could do away with the switch by using the fuse as switch - just get automotive fuses of the type that connects to spade terminals.

How would you use a fuse as a switch? This got me thinking about circuit breakers. Are there any issues that I would have to worry about with these? Will they wear out very fast over time though?

This got me thinking about circuit breakers. Are there any issues that I would have to worry about with these? Will they wear out very fast over time though?

You want me to Google data on endless versions of circuit breakers?Circuit breakers are simply SPST switches, they come in ranges from sub-1A to kA - which one do you think will last the longest?

Why do you want the switch/circuit breaker anyway?If it is for safety, like on Battle-bots, just use what they use (which is more or less a piece of copper connecting two terminals - remove it and all power is cut).Or, use the fuse that you need anyway.Or, remove the cable from the positive pole of the battery when you want it off (make sure the cable cannot push it into contact with the battery terminal when it's supposed to be off).

As long as the robot is powered, I don't see any purpose in switching the motor current off?

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives

I dont know. I guess I just don't fully understand relays. I understand that they are a electronically activated switch, but how do you activate them?

So if I wanted to use just one relay between the battery and the Sabertooth, I would just need a relay, a switch and a diode. If I were to use two relays in series (dont understand how that would work), would that double the amount of amps it can pass?

I'd like to get the relay from Amazon because I am already ordering a few parts from there, but I'm not sure if it will work for my circuit. Here is the components in my circuit; - two motors that draw 22 amps each- Sabertooth 2x25- DC-DC converter that reduces the voltage to 12 volts for all of the other electronics that draw a total of 4 amps at 12 volts.

I just want to verify this diagram with you guys before I begin to solder. The "bar" (cathode) on the diode goes on the positive terminal (terminal 85) of the relay, right? Here is my diagram.

Yes, the cathode (which is missing the bar in your drawing) goes to the positive terminal.Your switch should go to terminal #85 as in your drawing, however, your switch connects this terminal to ground!!! It should connect it to B+

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Regards,Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?Please remember...Engineering is based on numbers - not adjectives